_stop is a coroutine, so annotate it as one

Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>

hangupsthread.py

diff --git a/hangupsthread.py b/hangupsthread.py
index ba78d55..4cc4065 100644
--- a/hangupsthread.py
+++ b/hangupsthread.py
@@ -44,6 +44,7 @@
         if self.presence_timer is not None:
             self.presence_timer.cancel()
 
+    @asyncio.coroutine
     def _stop(self):
         yield from self.client.disconnect()
         self.loop.stop()